1. Field of the Invention
The present invention relates to a cover-gasket assembly obtained by direct formation and adhesion of a rubber gasket onto a cover member for a hard disk device.
2. Description of the Prior Art
A hard disk device generally comprises hard disks, namely, disks of an aluminum alloy or the like coated with a magnetic material which are enclosed in a sealed container, and magnetic heads each disposed close to the disk, with an about 0.1- to 0.5-.mu.m gap therebetween, for recording and reproduction of information into and from the disks.
If dust is generated in the sealed container or comes into the container from the outside, therefore, magnetic surfaces of the disks or the magnetic heads can be broken. In order to obviate such troubles, an air circulator comprising a high performance filter is installed in the sealed container to catch dust, whether generated internally or coming from the outside, thereby keeping clean air inside the container.
For preventing the penetration of dust or the like from the outside of the sealed container, however, any gap between a cover and a body of the container must be sealed with a gasket. Therefore, it is necessary for the gasket to have both cleanness such that the gasket itself does not generate dust or the like, and sealing properties sufficient for preventing the external penetration of dust or the like.
As the known gasket of this type, there have hitherto been gasket members molded from, or die-cut from a molded sheet of, neoprene rubber, urethane rubber, silicone rubber, nitrile-butadiene rubber or the like, and gasket members die-cut from foam sheets of these rubbers, polyolefin (e.g., polyethylene) or the like.
In use of these conventional gaskets for constructing a cover-gasket assembly, it has been a common practice to store the gasket with a release paper attached thereto through a pressure sensitive adhesive and thereafter adhere the gasket to a predetermined position of a cover member for a hard disk device by removing the release paper, or to adhere the gasket to the cover member with an adhesive. The use of a pressure sensitive adhesive or an adhesive has been the cause of such troubles as staining of the cover for the hard disk device, generation of dust inside the hard disk device, and the like.
In addition, accurate positioning in adhering the gasket member to a predetermined position of joint between the cover member and the container body of a hard disk device is extremely difficult to accomplish and requires skill, leading to higher manufacturing costs of cover-gasket assemblies.
Furthermore, production of the die-cut gaskets needs cutting dies, and inevitably generates waste sheets (cutting wastes) which cannot be utilized. Thus, there arise an added increase in production cost and difficulties associated with disposal of waste sheets.
The molded gaskets, on the other hand, not only need molds for the production thereof, leading to high production costs, but also have the disadvantage that an unsatisfactory deflashing upon molding will cause generation of dust in the hard disk device.
In consideration of the above, the present inventors have made a proposal to apply a liquid addition-curable type silicone rubber composition, as a gasket, directly to a joint portion of a cover for a hard disk device (Refer to U.S. Pat. No. 4,950,521 and U.S. Pat. No. 5,147,691). According to the proposal, the gasket is formed without using a pressure sensitive adhesive, an adhesive or the like; therefore, the problem of generation of dust inside the hard disk device can be obviated effectively. In addition, neither a cutting die nor a mold is used for obtaining the gasket, which is advantageous economically and offers freedom from the generation of cutting wastes, flashes or the like. Thus, the gasket according to the proposal is highly advantageous over the conventional gaskets.
However, in recent years there is a trend toward the use of hard disks with a sputtered magnetic film thereon and, in addition, the head-disk gaps have come to be set extremely narrow. As a result, there have been an increasing number of cases where troubles at head-disk gaps are caused not only by dust but also by bleedings, bloomings or condensates of volatile components coming out of the gasket material or the like. Of common gasket materials, silicone rubber is said to be the least liable to cause these troubles. Even in the case of silicone rubber gaskets, however, ordinary cured products of silicone rubber have the problem that low molecular weight siloxanes contained in the silicone rubber are evaporated and diffused in the hard disk device and are thereafter recondensed in head-disk gaps, leading to insufficient floating of the head above the disk surface and, eventually, to such troubles as head crash and destruction of magnetic disk surfaces.
Besides, silicone rubbers have comparatively high moisture permeability coefficients on the order of 100 g.mm/m.sup.2.24 hr, and are unsatisfactory as to sealing properties against permeation of water vapor. Therefore, use of a silicone rubber gasket for a cover of a hard disk device restricts the environments in which the hard disk device can be used.